In conducting chemical processes, it is regularly desired to monitor the level of liquid in the device in which such a process takes place. Whilst many methods exist to detect the level of liquid in a biphasic system of liquid and gas, this is less straightforward in circumstances where the liquid is present in conjunction with a supercritical phase. The supercritical phase, which is neither a strict gas phase, nor a strict liquid phase, is difficult to discriminate from the liquid phase. This problem becomes even more manifest, if (as will frequently be the case under the circumstances where a liquid and a supercritical phase co-exist in a chemical reaction), the liquid is a boiling liquid. Moreover, the detection of the level of liquid is particularly problematic in the event of reaction systems involving corrosive substances. A most prominent example of such a system is in the production of urea, by synthesis from ammonia and carbon dioxide. This reaction involves the formation of carbamate, which results in an extremely corrosive mixture of urea and carbamate in the urea synthesis section. This puts even more severe limits on the detection methods available.
The conventional method to date comprises radioactive measurements. The use of radioactive materials, however, comes with a plurality of drawbacks. These do not only concern the potential hazards, and required safety handling, of the radioactive material themselves, but also the social consequences of a negative public opinion, and the economical and regulatory consequences of authorities putting limits on granting permissions for the use of radioactive materials. These radioactive measurements are also very maintenance intensive. Hence, a technical solution is required that allows avoiding the use of radioactive materials in the monitoring of levels of liquid in a urea synthesis section, or in other systems where a liquid is present in conjunction with a supercritical phase.
Radar (originally an acronym for “radio detection and ranging”) is a well-known object-detection system which uses electromagnetic waves—specifically radio waves—to determine the range, altitude, direction, or speed of both moving and fixed objects. A radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter. Background art includes the use of radar to measure the level of liquids.
On the 11th Stamicarbon Urea Symposium (2008) it was proposed to use radar for the measurement of the level of liquid in a urea synthesis. The proposal entails the use of a horn antenna in the reactor and a standpipe in the stripper. The antenna serves to transmit a radio signal from a transmitter towards the media in a vessel, and to receive back echo signals that result from the radio signal encountering a reflecting target. For the horn type antenna it was conceived that the surface of the level of liquid in the reactor would constitute such a reflecting target, and the resulting echo would form a detectable signal.
However, in practice the method turned out to fail. Whilst the precise reasons cannot be easily established, it is clear that the ratio of signal to noise (S/N ratio) is highly unfavorable in systems wherein the interface to be detected is that between a liquid and a supercritical fluid. The S/N ratio is particularly unfavorable in systems such as a reactor for the synthesis of urea from carbon dioxide and ammonia, which not only involves the presence of a liquid and a supercritical fluid, but wherein the liquid itself will generally be boiling. This is all the more problematic since, particularly in the aforementioned synthesis of urea, the extreme corrosiveness of the reaction mixture puts severe limits on the availability of any systems for monitoring the level of liquid, let alone of systems other than those involving radioactivity.
Background art further includes a form of “guided radar” level measurement, used for measuring the level of a liquid in a container, such as a reaction vessel. The prior art technique operates by guiding radar pulses along a rod. This type of radar level measurement, along a duplex steel rod, is foreseen in the aforementioned Urea Symposium reference.
Background art includes WO 2004/046663. Herein an apparatus and method for radar-based level gauging is described, wherein a microwave signal is sent through a waveguide. The method is described for the purpose of measuring the level of a liquid, such as petroleum, in a situation where a gas, such as air, is present above the liquid. The disclosure neither addresses the specific situation of a liquid which has a supercritical fluid above it, nor the specific situation of a liquid which is present at the time of conducting a chemical reaction, under vigorous conditions. Rather, the disclosure is directed to level measurement of liquids in typically static situations.
Another background reference on the measurement of a level of liquid in a vessel, using radar, is US 2004/145510. Herein too, a static situation in a vessel is addressed, rather than at a dynamic situation when a chemical reaction under vigorous conditions is conducted in the vessel. Also, the reference does not address the specific measurement of a level of liquid in a situation where a supercritical fluid is present above said liquid.
It is now desired to provide a method for monitoring the level of a liquid in a system wherein both a liquid and a supercritical fluid are present. It is furthermore desired to provide a method for monitoring the level of liquid in the event that the liquid below the supercritical fluid is boiling. It is particularly desired to provide a method for detecting the level of liquid in a reactor for the synthesis of urea from carbon dioxide and ammonia.